1. Field of the Invention
The present invention relates to a method for annealing a single crystal of fluoride and a method for manufacturing a single crystal of fluoride. The single crystal of fluoride processed by the methods of annealing and manufacturing is suitable for use in lenses, prisms, and the like for an optical system of an excimer laser stepper, which requires a high precision image formation capability. More specifically, the present invention relates to a method of annealing and manufacturing a single crystal of fluoride whereby it is possible to obtain a single crystal of calcium fluoride, etc., which is appropriate for use in an optical system, such as a lens or window material, for various devices that utilize a laser in the ultraviolet wavelength range or the vacuum ultraviolet wavelength range, such as a stepper, CVD apparatus, or nuclear fusion apparatus. Also, the present invention is suitable for an optical system for photolithography with wavelengths of 250 nm or less (photolithography utilizing KrF or ArF excimer lasers, F.sub.2 lasers, solid-state lasers with non-linear optical crystals, etc.).
2. Discussion of the Related Art
In recent years, due to the progress towards higher levels of integration and higher complication in the functions of VLSI, micro-level processing technology on a wafer has increasingly been required. An exposure apparatus called a stepper, which exposes and transcribes the minute pattern of an integrated circuit onto a wafer made of silicon, etc., has been used in photolithography.
For the projection lens of a stepper, which is the key to photolithographic technology, superior image forming capabilities (resolution, focal depth) are required. The resolution and focal depth are determined by the wavelength of exposing light and the numerical aperture (NA) of the lens. For a fixed exposure wavelength, as the pattern becomes finer, the angle of the diffracted ray becomes greater. Therefore, if the NA of the lens is not sufficiently large, the diffracted rays cannot be collected to produce high resolution. If the exposure wavelength becomes shorter, the angle of the diffracted rays becomes smaller for the same pattern and, therefore, it would be acceptable to have a small NA for the lens.
The resolution and the focal depth are expressed by the following equations: EQU Resolution=k1.multidot..lambda./NA EQU Focal depth=k2.multidot..lambda./(NA).sup.2,
where k1 and k2 are proportional constants.
These equations imply that in order to improve the resolution, the NA of the lens (diameter of the lens) should be enlarged, or the exposure wavelength should be shortened. In addition, in terms of the focal depth, it is advantageous to shorten .lambda..
Recently, the wavelength of the exposure light ray has been becoming shorter and shorter. For example, steppers using KrF excimer lasers (wavelength of 248 nm) have already appeared on the market. There are very few optical materials that can be used in photolithography for light having a wavelength of 250 nm or less. Usually, single crystals of calcium fluoride and silica glass are used.
As for enlarging the diameter of the lens, simply having a large diameter (approximately a diameter of 150 mm to 250 mm) is not sufficient, and a superior homogeneity is required in the refractive index of single crystals of calcium fluoride (fluorite) and silica glass.
The single crystal growth of calcium fluoride has conventionally been conducted with a method called the Bridgeman method (Stockbarger, crucible lowering method). For a single crystal of calcium fluoride (fluorite) for small-sized optical parts and for window materials that do not require stringent homogeneity, the final products are obtained through finishing processes, such as rounding, after the ingot obtained from the crystal growth is cut out.
In contrast, for single crystals of calcium fluoride which require high homogeneity, such as those for use in optical systems (projection lens, for example) for a stepper, a simple annealing is first applied to the ingot and an additional annealing is conducted after an appropriate size of calcium fluoride is cut out from the ingot to produce the intended product. These anneals are necessary because the ingot has high residual stress and strain, as it is.
Because a single crystal of calcium fluoride reacts with oxygen at a temperature of 700.degree. C. or more, the annealing is conducted in an oxygen-free atmosphere. During this annealing process, the object to be annealed is enclosed in a container (container for enclosing the annealed product) made of materials such as carbon, which have a low reactivity at this temperature range, for example. Then, the entire container is enclosed in an airtight container which can be vacuumed.
By providing insulation from the external atmosphere with the airtight container, the annealing of the single crystal of calcium fluoride is conducted with an appropriate temperature schedule (control).
However, with the conventional annealing method, a haze may be generated inside the calcium fluoride upon annealing of the single crystal of calcium fluoride. As a result, the desired high transmittance cannot be obtained in such a case.
In addition, there has been the problem that by using the conventional annealing apparatus or annealing method for the annealing of a single crystal of calcium fluoride that is cut out into an appropriate size, it is impossible to obtain a single crystal of calcium fluoride with a satisfactory small strain (birefringence) (low strain within an acceptable range), which can be used for precision optical systems, such as excimer laser steppers.
Moreover, due to the enlargement of the diameter and the increase in mass of the single crystal of fluoride as an optical member, the problem in removing strain/birefringence (reduction of strain/birefringence) is becoming more and more difficult.